U.S. Patent RE32,600, issued Feb. 16, 1988, hereby incorporated by reference in its entirety, discloses a distillative cryogenic separation method employing a bottom additive stream. The method is directed to a hydrocarbon-containing feed stream, and is directed toward the production of overhead and bottom product streams, both to desired specifications. The method comprises recycling a portion of a bottom product stream, typically but not necessarily derived from the separation process, back into the upper portion of the column or into the reflux condenser. The distillative separation method described provides for a savings of energy and provides for the adjustment in the column operating conditions, such as the column operating pressure or temperatures. The distillation separation method employs a nonpolar liquid additive agent, typically a C4+ stream, that is, a stream composed of C4 or higher alkanes. This distillative separation method is particularly directed to the separation of methane and nitrogen from a natural gas hydrocarbon stream, and, in another embodiment, the separation of a carbon dioxide-containing feed stream to recover a C3+ stream, and is also directed to recovering a C4+ stream having lower amounts of recovered components, such as lower amounts of hydrogen sulfide, ethane, or propane therein.
The distillative separation method, employing the bottom additive as described, comprises a two-column system; the first column representing a high pressure recovery column, typically operated for propane plus (C3+) recovery, hydrogen sulfide removal, or ethane (C2+) recovery, and a lower pressure additive recovery column, operated to recover a C4+ stream and to permit recycling of a portion of the C4+ stream as a bottom additive to the high pressure recovery column. This two-column system, by its nature, requires a multitude of processing equipment, including, as a minimum, two distillation columns, two overhead condensing systems (condensers, reflux accumulator drums and reflux pumps) and two bottom reboilers. In addition, the two-column system requires a multitude of instrumentation for control and system monitoring.
It is therefore desirable to provide for a new and improved distillation separation method employing a bottom additives stream. The method, as described, is particularly directed to, but not limited to, the propane recovery separation.